Earlier quoted context omitted.
Do you think GDP is the critical factor for determining a global reserve currency? IMO, respect for property ownership rights, legal/judicial recourse, ubiquity of US banks, and political stability are large contributing factors that few other countries can compete with. China has a pretty checkered past regarding ownership rights for citizens who are critical of its leadership.
I suspect it's a prerequisite, though perhaps not the only one. I also suspect that political stability is more likely to favour China than the USA over the next few decades — it is very human for success to be followed by the assumption of indefatigability and that to be followed by fighting over who leads the nation, rather than continuing to focus on what actually made a nation dominant and how to keep it that way…
As for the US, they're not alone in this but they are now in a situation where younger voters with a longer time horizon are able to win majorities against older voters with shorter time horizons. This means they're entering into a period of creativity. It is not clear what the outcome will be. But it is clear that we cannot take for granted that pursuit of an unreformed ideology will distract them from the compromise and teamwork that brought them to strength.
In both cases, I think you're extrapolating from recent politics. But both countries are in different turns of the demographic wheel, and have different ways of dealing with that. Democracy also tends to make turmoil more obvious to external observers, so it's easy to accidentally compare apples to oranges when you're looking at a democracy and an autocracy.